1. Field of the Invention
The present invention relates to an ion generator and an ion generating method.
2. Description of the Related Art
In a semiconductor manufacturing process, a process of implanting ions into a semiconductor wafer has been implemented on a standard basis for the purpose of changing conductivity, for the purpose of changing a crystal structure of the semiconductor wafer, and for some other purpose. An apparatus used in this process is generally called an ion implanter.
As an ion source in such an ion implanter, a direct current (DC)-discharge type ion source is known. A DC-discharge type ion source heats a filament by a DC current to generate thermal electrons, and a cathode is heated by the thermal electrons. Then, thermal electrons emitted from the heated cathode are accelerated into an arc chamber and collided with source gas molecules introduced thereinto, to ionize atoms contained in the source gas molecules.
For the source gas molecules to be introduced into the arc chamber, a halide such as a fluoride or a chloride is often used. The source gas molecules of the halide generate halogen radicals in an ionization step, and the halogen radicals act on a component constituting the ion source, such as a metal material for an inner wall of the arc chamber, to form a chemical bond. Then the chemically bonded metal material is easily vaporized and ionized along with the source gas molecules, and extracted as an ion beam from the ion source with the desired ions.
This might result in that the metal material for the inner wall of the arc chamber is implanted as ions into a semiconductor wafer, leading to contamination of the wafer by impurity ions of the metal or the like. Especially in the case of the DC-discharge type ion source, a high melting point metal such as molybdenum (Mo), tungsten (W) or tantalum (Ta) may be used for the inner wall of the arc chamber since a temperature in the arc chamber becomes high. Metal contamination by such a high melting point metal is desired to be reduced as much as possible for improvement in performance of a semiconductor device.
Accordingly, there has been considered a configuration where carbon is used instead of the foregoing high melting point metal as a member for the wall of the arc chamber. However, highly reactive radicals, originated from the halide generated in the ionization step, easily cause wear of carbon. Further, when the carbon ions and neutrals having scattered are deposited onto arc chamber and beam extraction parts and becomes a conductive film, it brings about an insulation failure. Therefore, life of the ion source becomes significantly short, and the productivity of the implanter deteriorates.